1. Field of the Invention
The present invention relates to an optical pickup apparatus that performs an operation of reading out a signal recorded in an optical disc or of recording a signal in the optical disc by use of a laser light.
2. Description of the Related Art
An optical disc device is widely used that is capable of performing an operation of reading out a signal or recording a signal by applying a laser light irradiated from an optical pickup apparatus to a signal recording layer of an optical disc.
While the optical disc device using the optical disc called CD or DVD is generally in wide use, recently the optical disc device has been developed that uses the optical disc with improved recording density, namely, the optical disc of Blu-ray standard or HD-DVD (High Density Digital Versatile Disc) standard.
The laser light used for the operation of reading out the signal recorded in the optical disc of the CD standard is an infrared light with 780 nm wavelength and the laser light used for the operation of reading out the signal recorded in the optical disc of the DVD standard is a red light with 650 nm wavelength.
As opposed to the laser light for such optical discs of the CD standard and the DVD standard, the laser light used for the operation of reading out the signal recorded on the optical disc of the Blu-ray standard or the HD-DVD standard is a laser light with a shorter wavelength, for example, a blue light with 405 nm wavelength.
A protective layer provided over the signal recording layer in the optical disc of the Blu-ray standard is 0.1 mm thick and the numerical aperture of an objective lens used for performing the operation of reading out the signal from this signal recording layer is set at 0.85.
On the other hand, the protective layer provided over the signal recording layer in the optical disc of the HD-DVD standard is 0.6 mm thick and the numerical aperture of the objective lens used for performing the operation of reading out the signal from this signal recording layer is set at 0.65.
As described above, since the blue light with 405 nm wavelength may be used as the laser light for performing the operation of reading out the signal recorded in the optical disc of the Blu-ray standard and the HD-DVD standard, the optical pickup apparatus capable of performing the operation of reading out the signal from the optical discs of these two standards may be produced by using a laser diode commonly for both usages.
However, since these two optical discs have their signal recording layers provided at positions with different distances with respect to each other from a laser light incident surface, the distances from the objective lens to the signal recording layers greatly differ between these two optical discs. Since both optical discs are largely different in numerical aperture required by the objective lens as described above, the numerical aperture must be switched corresponding to each optical disc, to read out the signal from both optical discs. There has been developed the optical pickup apparatus that is capable of performing such operation (see Japanese Patent Application Laid-Open Publication No. 2006-172605).
The optical pickup apparatus conforming to the optical disc standards with such improved recording density is required to meet severe optical characteristics for enhancing signal recording quality in accordance with the improvement of the recording density.
The optical pickup apparatus is configured to be capable of controlling a driving current supplied to the laser diode so as to obtain laser power suitable for reading out the signal in the optical disc or for recording the signal in the optical disc.
The optical pickup apparatus is configured to be capable of performing a control operation of focusing the laser light irradiated from the optical pickup apparatus on the signal recording layer of the optical disc, namely, a focusing control operation, and a control operation of causing the spot of the laser light to follow a signal track, namely, a tracking control operation.
In the optical disc apparatus, while the optical disc is driven to rotate when placed on a turntable driven to rotate by a spindle motor, a tilt is caused to the optical disc due to a warpage of the optical disc itself, the state of placement of the optical disc on the turntable, and a mechanical error. When the optical disc tilts, the angle of an optical axis of the laser light relative to the signal surface of the optical disc deviates from the optimum state.
Since the deviation of the optical axis of the laser light relative to the signal surface of the optical disc described above becomes a great problem in accordance with an increasingly higher density of the signal recorded in the optical disc, an optical pickup apparatuses in recent times has been developed to include a tilt coil capable of performing an operation of adjusting the tilt of the optical axis of the laser light, what is called tilt controlling operation.
The technology described in patent reference 1 described above makes it possible to manufacture the optical pickup apparatus for performing the operation of reading out the signal recorded on the optical disc of the Blu-ray standard and the HD-DVD standard. However, the optical pickup apparatus has a problem that occurrence of: spherical aberration due to the thickness, etc., of a cover layer between the laser light incident surface and the signal recording layer of the optical disc; and the aberration called coma aberration prevent the signal reproducing operation and the signal recording operation from being normally performed, and the technology has been developed for solving such a problem (see Japanese Patent Application Laid-Open Publication No. 2006-147069).
In the technology described in Japanese Patent Application Laid-Open Publication No. 2006-147069, it is possible to correct the spherical aberration and the coma aberration, however, there is a problem that configuration must be such that an aberration correcting element, which corrects both of the aberration, is provided in the light path leading the laser light emitted from a laser diode to the objective lens and also is displaced in the two-axis direction to correct the coma aberration, and the complication of the structure of optical system is unavoidable and therefore this technology is not suitable for the optical pickup apparatus for which accuracy is required.